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BLEED FAULT lamp illumination under various conditions
In lots of commercial and defence aircrafts, bleed fault is annunciated because of mainly the following reasons:

Overpressure/Overheat
shut-off valve disagreement (i.e. failed open, failed closed)
pressure regulator shut-off valve fails
Precooler fails
Computer failure

However, when there is an emergency condition e.g. engines are lost/off/unavailable (i.e. no generators) and also no APU possible, should there be any bleed fault? Given that the aircrafts are only running on batteries, should there be a bleed fault because your sensors and computers aren't powered up completely?
I know that you have things like built-in autotest stuff that goes on before the computers are powered, but surely when the aircraft is on batteries/emergency power, should there be any bleed faults?

At the moment this question seems to be too broad and cannot be answered objectively.

@yankeekilo        Thanks for your comments

Perhaps you can edit the question and add more information to make it more specifc (and easier to answer)?

The question is quite broad but here's my understanding..

However, when there is an emergency condition e.g. engines are lost/off/unavailable (i.e. no generators) and also no APU possible, should there be any bleed fault?

No, since there is no fault (as you've listed). The bleeds are in the correct config and there is no disagreement between the valve and the switch. There is just no pressure to the system. You will get a 'Bleed Off' indication if your switch is on with no pressure to the valve(type dependant). Also you will get a pack off indication - if you've got the bleeds set to feed the packs.

I know that you have things like built-in autotest stuff that goes on before the computers are powered, but surely when the aircraft is on batteries/emergency power, should there be any bleed faults?

As I said above you wont get faults, but you may get indications that the bleeds are off.


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Do stealth airplanes need to limit contrail generation?
Stealth aircraft have low visibility on radar.
I assume that they strive to achieve low visibility optically as well, e.g. spend some effort to not stand out too much visually from the surroundings.
A condensation trail (during daytime against a bright sky) could definitely make an invisible aircraft visible.
So what do stealth aircraft do against condensation trails?
Do they have technology aboard to avoid or minimize them?  Or it just a matter of choosing the right conditions (clouds, humidity, altitude, temperature) to fly?

wikipedia only mentions choosing right altitude

@ratchetfreak and the WikiPedia article about stealth aircraft does not mention contrails at all.

USAF Brevity Codes include words like 'marking' to designate an aircraft that is conning. Typical procedure for most US fighters is to avoid flying at an altitude that results in marking.

Some nice info here: http://aviationweek.com/StealthTech#slide-3-field_images-1351991
"Contrail suppression was demonstrated in the 1960s by the U.S. Air Force, using chlorosulfonic acid (CSFA) injected into the engine exhausts."

Do stealth airplanes need to limit contrail generation?

Not just stealth aircraft, most military aircraft are required to avoid contrails. Contrails form due to moisture in the aircraft's exhaust. A tried and tested method by NASA is NOT to fly in regions of air that support contrail formation.
Ophir's Pilot Alert System which is used by the B-2 stealth bomber uses LIDAR (light detection and ranging) to differentiate contrails from clouds and tells the pilot to change his altitude as/when necessary.
That being said, it is essential to remember that though contrails are visible to the naked human eye they are NOT visible to Radars. Ergo, until the stealth aircraft is really close (assuming it is on a SEAD/DEAD mission) one cannot see the contrails and by the time they do, it is too late. Moreover, stealth aircraft generally operate during night when contrails are less visible.

Stealth aircraft have low visibility on radar.

Not necessarily. VHF radar should be able to detect them at long range. Moreover, RCS of aircraft is a complex phenomenon, depending on many stuff (frequency, aspect angle, polarization of signal etc). You will never find a "static" figure for this.

not to forget their infrared signature. A high flying B-2 can be spotted from 400 km distance with a good IR sensor due to the heating of the leading edge (just a few degree, but very hard to avoid). And that is long before the airplane arrives, not when it is almost overhead.

"one cannot see the contrails and by the time they do, it is too late" .. not true for all cases. For example -> the jet(s) are flying to a far off target (reactor? ) and need to avoid detection for quite some time. In the meantime, if some farmer on the ground spots the jets early and gives warning, the target may have time to prepare resistance.

Leave alone a farmer without the help of an acquisition radar it is impossible to differentiate between jets that are flying at altitudes of 20,000 ft plus.Fighter aircraft(including stealth) are NOT supposed to fly very long distances.They are based close to the theater, so that they can react quickly.Countries like the U.S, China, Russia who have a constellation of satellites at their disposal can very easily track a Stealth aircraft from the moment it takes off, contrail or no contrail.

@D_S: Who said anything about fighters, though?  The B-2 stealth bomber is reported to have a range of 6,000 nm, and its Wikipedia article describes a number of missions where it flew nonstop from the continental US (with in-flight refueling) to attack targets in Africa, Central Asia, and the Middle East.

Stealth aircraft include options to reduce visual signatures, so yes.
This is achieved as a side effect of reducing the infra red signature, which is done by mixing the engine exhaust with environmental air before it is ejected, cooling the air a lot and thus making contrail generation far less likely.

actually some option include heating the air so the trail that forms evaporates again more quickly

I also find it counter-intuitive that making air colder would avoid condensation.

The cooling of the air is to lower the IR signature of the engines.  This makes it harder for IR radars and heat seeking missiles to track the aircraft.  I am unaware of air cooling having any effect on contrail generation which is generated by pressure changes in a high humidity environment.

"cooling the exhaust" does not mean down to condensation temperatures - it means lower than the 1500+ degrees of normal jet exhaust.

@paul true, but lowering the temperature will help, even if not 100% effective. They sometimes also inject chemicals into the exhaust. Also not 100% effective but it helps.

@jwenting Can you clarify how lowering the temperature would help avoid condensation?

@florisla condensation happens when higher temperature gasses rapidly lower in temperature on contact with colder surfaces (like particulate in the air). The same gas at that lower temperature would not condense on that surface. That's why your car gathers condensation during the night (when it cools more rapidly than the air around it) but not during the day (when it's at the same or higher temperature than the air around it)

To directly answer your question, stealth aircraft have no special equipment to limit their contrails.  The military does though incorporate atmospheric data into their flight planning to minimize the creation of contrails especially during daytime operations.  This is true not just for stealth aircraft, but any aircraft which is concerned about being detected visually (such as reconnaissance aircraft, etc).
NASA has a website that projects the formation of contrails.  Essentially it looks at the relative humidity with respect to ice (RHI) in the atmosphere, and if it is greater than 70% the region is flagged.  Areas with greater than 100% RHI are very likely to support the formation of contrails.

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Why does a helicopter need cyclic control rather than just tilting the whole main rotor?
Helicopters use a cyclic control to tilt each blade differently through the rotor's revolution. This is quite a complex mechanism. What is the advantage of this vs tilting the whole rotor plane? This seems to be a much more simple solution.

If the helicopter had any forward speed, wouldn't the lift of the rotors heading into the wind would be greater than those retreating?

I sure wouldn't want to get near one on the ground that had a tiltable rotor running.

@MarkAdler Does the Osprey count?

Google for "dissymmetry of lift".  The blades have "flapping" hinges which enable them to flap up and down and twist.  The advancing blade(s) flaps up, reducing the angle of attack and therefore the lift produced, whilst the retreating blade flaps down, increasing the angle of attack and therefore lift.  This counters the tendency for the helicopter to roll towards the retreating blade.  Ultimately, it is this phenomenon that limits the maximum speed of a helicopter.

@DavidRicherby No, it doesn't.

Tilting the rotor doesn't do much, in fact the body will be tilting more than the rotor. The gyroscopic effect will work against you.
The cyclic increases lift on one side of the rotor which will tilt the rotor. Which will then move the craft in the correct direction.
In other words the cyclic system gives a more direct response to input.

Rachet Freaks answer is good, but I feel a bit incomplete.
You ask why it needs a cyclic control.  Well, you need a control to tilt the disc, either directly or by flying it into position, so let's call that control the cyclic control (since it affects the rotor cyclically).  So whatever your mechanism, you need a cyclic control to tilt the disc.
So what do we mean by tilt the disc?  It's splitting hairs a little bit, although precise, to distinguish between tilting the disc, and causing a cyclic change in lift to each blade such that the rotor flies into a tilt.  The reasons for flying it into a tilt are as Rachet Freak states.
You then state that it's complex, so let's examine physically tilting the disc versus cyclically altering the pitch of each blade.
Cyclically altering blade pitch:  The power shaft from the rotor gearbox passes through the middle of the swashplate and is attached to the rotor.  The bottom of the rotating rotor is the rotating part of the swashplate which acts as a bearing surface rotating on the lower part of the swashplate which is non-rotating and fixed to the fuselage.  The blades are attached to the rotating swashplate with pitch links which can move to turn the blades in their cuffs.  Now all we need to do is to attach the cyclic control to the non-rotating swashplate so that we can tilt it.  In turn, this will tilt the rotating swashplate and the pitch links will transmit this to the blades, causing them to rotate in their cuffs, altering the angle of attack and therefore lift.

(By KoenB (blender drawing) [Public domain], via Wikimedia Commons)
Physically tilting the disk:  The first half of this is the same.  Just remove the pitch links so that the disc will physically follow the swashplate.  All good so far, and the only complexity you have removed are the pitch links attached to the blades to rotate them.  But now you have a problem.  You can only increase or decrease total lift by varying the power applied to the rotor from the engine and therefore the total rotor thrust.  The easiest way to solve this is to put the pitch links back and allow the blades to rotate in their cuffs.
Therefore, trying to physically tilt the disc needs the same complexity as cyclically varying the pitch and loses a lot of the utility of the helicopter.

I guess Gyro vanilla tillting device is a good illustration of the second option.... Tons of (public domain) pics available on the net I presume?!

The earliest autogyros by Juan de la Cierva did this, tilt the whole rotor. But when the autogyros became bigger, the control forces became too heavy and that is when he designed the feathering mechanism, so that aerodynamics could take care of it by using blade flapping.
It's actually quite beneficial to not have to tilt the whole rotor: the rotor axis is beefy and it has gearboxes connected to it. Yes the feathering mechanism is complicated, but a construction to allow the rotor axis of let's say a CH-53 to tilt, with its three engines and gearboxes connected to it, has its own complicated problems to solve.

Image source

Also for high angle of attack, tilting rotor is not possible

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When can I request flight following?
I'm a student pilot doing all VFR flying and making flight plans for training. I've heard that most people don't file a flight plan for VFR outside of training though, and instead request "flight following" from nearby ATC.
When can I do this in flight? Which ATC can do this for me?

It's worth noting out that flight following and VFR flight plans are complementary services with different goals. VFR flight plans provide automatic search and rescue if you should go down. Flight following alerts you to nearby traffic, with no search and rescue guarantees. (If you drop off the radar while on FF, the controller might just assume you're in an area without radar coverage or had a transponder failure.) I recommend doing both.

As SkipMiller said, you can basically request flight following at any time and any certificate level - student, sport pilot, private pilot - the service is available to anyone.
Also:
We're told that FF is "workload permitting".
I used to think that meant they'd give you services when they can.   But I was wrong about that. Instead, once they accept you, you're in the system, and get full services.   If they get too busy, they'll tell you and kick you out of the system.  ("Radar Services Terminated; Squawk VFR")
FF depends on radar services.   When flying rather low and near mountains, its easy for them to lose you.
As for when you should request it:  You can request it at anytime.
Personally, I wouldn't be in a hurry to request it as soon as you depart.   Wait until you're established on course, in a cruise-attitude (instead of a climb), and everything is settled and running smoothly.  Then know your position, and how you're going to report it.  Know where you're going (both by name and ICAO identifier), and try to anticipate any questions they might ask.
Look for the Center/Approach/Depature where you are  (look in your AF/D for an airport close to you), and call up with:
You:   "Seattle Center; November 12345: VFR, with request"
translation:  Calling Seattle Center, this is N12345.  I'm VFR (so you haven't heard from me before... don't be surprised!), and I have a request.
Center:  "November 12345: Say request."
You:  "November 12345 is SkyHawk, 10 mi. South of Harvey Field, 4,500 feet.  Request VFR Flight Following to Friday Harbor"
translation: I'm a Cessna Skyhawk, my location and altitude will help you find me.  I'm asking for FF to this other airport."
Center:  "N345: Squawk 4532 and Ident."
You: (Set your transponder to 4532, press the Identify Button), and say "4532 and Ident, N345"
Center:  "N345: Radar Contact, 11 mi. south of Sierra Four Tree, at 4,500.  Traffic at your 1o'clock, 5 mi. North Bound, Altitude indicates 3,000."
translation:  I found you on my radar scope, and am describing the position I see you. (If you disagree, speak up!).  You have traffic ahead and slightly to your right.  Their transponder indicates 3,000 feet, but I cannot independently verify that.

I'll add that "workload permitting" in busy airspace can be greatly effected by how competent you sound on the radio.  When getting vectors to KEWR in the NYC class B, I could always correctly guess who did and didn't get ATC services based on their initial call.

If I were a student pilot on a solo XC I would ask for FF as soon as I switched from tower or departed. If the departure airport underlies class C or B, I would ask ground for a code before departing.

Workload permitting means that they can effectively not provide services without terminating you.

@mongo:  I have been told exactly the opposite:  That once you are under FF, you are receiving services, and if they are unable or unwilling to provide full services, they will terminate you.  Providing limited or no-service without a "Cancel FF" notice is not an option.

@abelenky, do you have an authority regarding your statement about limited service or interruptions of service for VFR aircraft? Because that is contrary to hundreds of personal experiences.  It doesn't happen often, but it does happen, and does happen deliberately.  I would love to see an authority to the contrary!  Thanks.

Know where you're going by name and/or FAA Location ID. Please DO NOT give a TRACON or tower/ground controller the ICAO identifier, and for that matter don't give a four-character FAA LID on initial callup either. The radar scope only accepts three-character identifiers.

Since a student pilot posed the query, here are some various thoughts based on my experience with FF. I'm a low-time VFR pilot (500 hours). I rarely make short flights; mostly cc for 1-3 hours.  (I wouldn't typically use FF for a 30-min hop). I consider FF a HUGE privilege that could easily be taken away if enough GA pilots abuse it, neglect it, or don't take it seriously.
I always use FF at night.  I highly recommend that.  I do a lot of night flying.
If I'm flying thru or even near Charlie airspace by day, I use FF near and thru that region.
If I'm flying near or thru Bravo airspace by day, I use FF for the entire flight.
I have never been refused for FF. Maybe it required a 2nd request a few times, but never refused.
Make sure your transponder is kept in good working order and calibrated according to your AD once you get your own plane.  If there is an issue with your transponder, FF may ask you to ident, and if they still can't establish a fix on you, they may ask you to recycle your transponder (turn it off, give it 5 seconds, turn it back on, let it re-set.)  Never do this unless asked or without asking their permission.  You drop off their radar completely for several seconds, and if they're not expecting it they'll rightfully freak.
It's very easy to miss a call from FF if you're talking with your passengers.
I inform my passengers that I'm using FF and I either need to isolate myself or limit discussion.  Most of my passengers love to hear the ATC chatter.
Don't refuse a "request" from a controller unless it creates a safety concern. For example, I've been kindly requested to veer around a busy corridor near Louisville at 11 pm when UPS has taken over the airport.  (Why would I say no?)  Understand if you're not in the corridor or in the airspace, FF will "request" rather than "demand."  Tell them you'd be happy to.  It will probably cost you 90 seconds. Remember you're the smallest guy they're watching.  Respect that!!
Whenever you hear your tail number called, put your right hand on the frequency dial before they say the next word.  Nothing annoys FF more than low time VFR pilots asking two or three times to repeat a frequency when they are handing you off to the next sector.
Learn which FF calls MUST be answered and which don't.  You'll probably learn mostly by trial and error, but you can get in far more trouble by saying nothing than by offering a quick, "937 Affirm."  When they say, "937, traffic at 10 O'clock, 7,000 feet. No factor." you don't have to reply ... but why not?  "937 Roger."  It's a lot like being reassured that your teenager didn't fall asleep in the middle of a conversation.
I never fail to affirm a frequency change on a handoff ... I don't necessarily read back the new frequency ... just "937 Affirm.  G'night."  If they go looking for you, they have reason to be annoyed.  I've been the "annoyer" before.
Always help a controller who is asking you to help find a missing pilot.  They'll love you for it.  (If you ID yourself as a student, they likely won't pick you to ask for help)  That's reason #267 to always keep a pen handy ... a semi-panicking controller who says, "937, can you do me a favor?  Go to 123.475 and see if you can find N654 Mike Papa.  He never terminated radar coverage and we can't find him."  It happens.  More than you'd think.  Drives the  controllers crazy.
ALWAYS advise on your altitude changes.  They'll usually say, "Maintain VFR and advise of any altitude changes."  Again, it will drive them nuts if you start a descent without advising.  When you advise, they will always verify and usually thank you.
If you're using an intermediate waypoint such as an airport or VOR, let FF know it if it's far enough off of a direct route to your destination airport that it could cause some confusion.  04º ... don't bother.  25º ... bother.  Same goes for deviating around weather.  Chances are they'll let you know of the approaching weather before you see it, but do all you can to make their job easy.  Beat 'em to the punch and they'll appreciate it.
Once I'm within 15 miles or so of my destination airport, I often get the feeling that FF is reeeeally wishing I'd terminate radar so they can get rid of me ... but since a lot of confusion and bad things can happen on approach, don't terminate FF earlier than you feel comfortable.  They'll want you to tell them you have the weather and the field in sight when you terminate.  Whether it's legal, proper, assumed, or otherwise, when you advise, "Indy Service, N937 has the Indy Metro weather and field in sight" they will immediately respond with their "termination" spiel.  By day, I typically do this 8-10 miles out.  By night, especially with any weather, rarely before 5-6 miles out. When you start flying RNAV/GPS or ILS approaches, that's usually the intermediate approach fix.

While this is incredibly useful, it doesn't really answer this specific question. It might be best for you to ask and answer a more general question about flight following.

5 seconds is no longer necessary unless you still have a cavetron in your txp. If you drop off radar for 5 seconds is no biggie. The radar system will go into "coast" mode that projects your position in the absence of a txp signal.

@WildFlyer Good post, but you got a couple of things wrong. ATC doesn’t give you suggestions or requests. If they tell you to do something you must do it. If you don’t want to do what they say, then don’t get Flight Following. Also, always acknowledge an ATC radio call. If you don’t they’ll just have to repeat it. I always let them tell me to contact the tower. The tower knows who you are and is ready to fit you into the sequence. At an uncontrolled field, they will often let you know how many other planes are in the pattern or approaching the airport, so I never drop FF but let them drop me.

You may request flight following anytime, pre-certificate and after.  My CFI required me to request FF while on my student solos - and sometimes ATC is too busy to grant that service, so don't be too surprised if you are turned down.  It is always based on a workload permitted basis by ATC.
Another tip - while you are a student pilot, always identify yourself as a student in your flight following request.  ATC does in fact go out of their way to assist students.
Nowhere is this written down, but I would not request FF for a very short flight.  To me that is just annoying to the controller.  But that may just be me.

I would request flight following on most flights, short or long as soon as practical, which means, on the ground (with ground control) or if at an uncontrolled airport after I have finished flying the plane safely out of the pattern and started navigating to my destination (even if I am not at cruising altitude yet).  If you talk to ATC and they are too busy, they will let you know that they are too busy, But most air traffic controllers  would much rather have radio contact with you, then not be in radio contact and try to guess what you are doing.  For example you are flying vor outside their controled airspace and are ascending to your planned flight altitude, mean while, a wide body commercial plane is descending across your flight path.  It is easier, quicker and safer to talk to you about keeping your current altitude until advised, or change your heading then have to divert a wide body plane around you which is on an IFR flight plan because you did not see them.
You get the added safety of another set of eyes looking out for you (some of the time as work load permits) and they get communication with you and an ability to move you around to increase safety.  They get another piece on the board that they can legally control, that they normally have no control over.  It is a Win for both sides.


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Why is "NO STEP" used instead of a grammatically correct spelling?
There are commonly "NO STEP" signs on airplanes where damage could be caused if a maintenance worker inadvertently put weight on a delicate part of the fuselage.
However, the grammar of this appears to be off. It should either be:

No Stepping -- akin to No Smoking, as it is the activity that is prohibited

or

Not a Step -- this is not a step (noun)

Does anyone have any sense of why the grammar of this sign is incorrect? What is the history of it? Does it hark back to the military?

My gut feeling is that this could be more suited for English.SE and their etymologists, rather than aviation. Most likely, and this is speculation now, they used NO STEP because it was short, easy to understand and made the message come across. Don't other signs also use incorrect grammar, e.g. "No turn on red"?

I lived in non English speaking countries and this kind of grammar mistakes are usually easier to understand by people. Plus, it's shorter so easier to read/to put in big caps. My guess it that it is correct and that the rest of the sentence is simply omitted.

I'm voting to close and migrate this question to English.SE.

Sometime you have both "no step" and "no stepping".

To be honest, although is a language related question, I don't think it is off-topic, as might give us better understanding of small details of aircraft business. I recommend not to close.


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How would you explain Bernoulli to a nine year old?
This is a aviation principles question coming from my nine year old daughter that I need help with. I was explaining to her the principle of flight and the fact that you need airflow under the wing in order to create lift and that this is why an airplane in a steep climb will stall etc. So she said, why not have two large fans that blow air across the wings (we are not talking about the propellers here, but rather an external fan to create air-flow) so there is always un-interrupted airflow!!!! I am not sure exactly how to answer this. The best explanation I could think to give her was just as a sail boat cannot sail simply by placing a big fan on the boat.....but I don't think she was convinced.

Actually an airboat is kind of a sailboat with a big fan.  (Well, technicall no sail either, but hey.)  An aircraft can fly with just a big fan too, but it's called a helicopter.

Something very similar to this is actually used in real life: check out blown flaps.

See also: http://aviation.stackexchange.com/questions/466/how-do-i-explain-what-makes-an-airplane-fly-to-grandma

While the question is good, it's got very little to do with Bernoulli's principle.

Also note that a sailboat can move by placing a big fan on it and blowing at the sail. The airflow from the fan bounces off the sail and creates thrust, just as if you'd have turned the fan backwards. It's not very efficient, but it works. (Mythbusters have an episode on it too, if you want to see it in action)

A helicopter is not a good example for this because the spinning rotors create lift by reaction on surrounding air, not by directing the airflow over a stationary wing.

Stick your thumb over the nozzle on the garden hose.

I think this is the best experience to explain lift : https://www.youtube.com/watch?v=6TDT2h7-ycY

The Antonov An-72 "Coaler" actually uses this principle: https://en.wikipedia.org/wiki/Antonov_An-72

One answer is that that would actually work. In fact something called Boundary Layer Control BLC) was initially used on the F-4 to increase lift on the wing and lower the stall speed a bit. The problem is that the hardware to do this sufficiently to create enough lift to hold the aircraft aerborne would be prohibitive. Check out what is called  (https://en.wikipedia.org/wiki/Magnus_effect) or https://www.grc.nasa.gov/www/k-12/airplane/cyl.html

But I would not imply (to your point about aircraft in steep climb stalling), that lift, or stall is in any direct way related to an aircraft's Pitch attitude. It is the Angle of Attack, (AOA), which, no matter what the pitch attitude, bank angle, gross weight, or airspeed, that causes lift and/or stall.

"So she said, why not have two large fans that blow air across the wings (we are not talking about the propellers here, but rather an external fan to create air-flow) so there is always un-interrupted airflow!!!!" Are the fans attached to the aircraft? If so, aren't an aircraft's propellers "external fans designed to create air-flow"? If not, then the answer is trivially obvious.

It works, but it is just the wings. For a controlled flight, we'd need two big fans for the wings (one for each), two smaller fans for the elevators, and one more for the rudder.
So that's 5 fans for each airplane.
The concept of controlled flight can be demonstrated by hanging two strings to any model airplane (plastic ones recommended, model airplanes stall just like real planes...). If we want to pitch up or down, we would need at least 3 strings.
Another consequence of having the fans blow air backwards across the wing is, due to action and reaction, there is a thrust pushing the plane forward. That's not a bad thing, planes are designed to move forward to get from A to B, not create lift vertically then settle down exactly where it started.
So the 5-fan plane will move forward anyway. Engineers figured out it is better to have one fan, called the "propeller", at the nose of the plane instead of 5.

Not "just for the wings". If you move the air sufficiently fast enough over the top surface of any object, it will reduce the static normal (Perpendicular) pressure and cause Lift. Indeed, see links in my comment above, even a rotating cylinder causes such a [pressure differential.

Bernoulli's principle is probably not the best starting point to explain lift generation, especially if you do not first explain the origin of the Bernoulli effect! Naïve invocations of Bernoulli have led to a number of incorrect 'explanations' that are quite widely circulated, such as the idea that air flows faster over the upper surface of a wing because it is more curved that the lower, and thus creates a longer path. If this explanation were correct, paper airplanes and inverted flight would be impossible.
Bernoulli's principle is a consequence of Newton's laws of motion applied to incompressible fluid flow. To explain lift, you can start by invoking Newton directly, by observing that lift is the reaction to the wing's downwards acceleration of the airflow around it. Bernoulli enters the picture when you get to calculating the changes in pressure and velocity within that airflow.
From this perspective, we can see that while blowing air over a wing would create some lift, as the wing will deflect at least some of that airflow downwards, you would get more lift by pointing the fan directly downwards. This is, of course, what a helicopter does, and it takes a lot of power, so wing-blowing, being a less-effective way of generating lift, would require even more. If, instead, you use the fan as a propeller, you only have to produce enough thrust to counter the drag, which may be as little as 1/50 of the lift.

Great answer, but I don't think a 9 year old would understand it...

@Danny : You could go with "yes, it would create lift, but it is not the most efficient way of doing so, and would take a lot of power to get airborne - more than a helicopter of the same weight."

One point that I didn't include originally is that this mechanism would produce a lot of thrust as a side-effect. That would make landing difficult.

@Danny : I should have added that this particular 9 year old seems pretty sharp.

Blow over the top of a piece of paper and it will rise up. It rises up for the same reasons as a wing does.

...which has no downward acceleration of air at all...?!?

Of course the paper causes downward acceleration of air.  Like germs, just because you can't see it doesn't mean it isn't there.

Not that it is the cause. The downward acceleration is the consequence of the aerodynamic forces involved, not the cause. How could it be the cause? It happens after the air leaves the paper.

Drive on the freeway. Tell her to stick her arm out the window and pretend it's an airplane wing. That will be a great jumping off point for explaining how when the air flows over her arm, it creates areas of high and low pressure, which causes lift.
To explain high and low pressure to her, (which is a whole other can of worms entirely) bleed air from a bike tire. Also opening a soda can would help.
Also it sounds like you need to explain Newton's Laws (or at least the third one). There are countless examples of this in every day life. You walk on the ground, the ground supports you, etc, etc...
Everyone learns differently. Personally I'm an advocate of learning by experimentation, trial, and error. Try to find new ways to let her figure out the physics concepts on her own through experimentation, and then provide the academic understanding.
Wind, lift, flow, and fluid mechanics are very broad concepts which require simplification for many years into the learning process. Which is what learning is, a process. Start small, build gradually, and you will see results faster than you think.
Experience: I've been a student for most of my life, so I know a lot about not knowing much.

As far as airfoils go, hands/arms are pretty much symmetric, so while there will be changes in pressure due to Bernoulli's principle, it'll be symmetric and no lift is generated (due to Bernoulli's principle, angle your hand and there will still be lift though). While sticking your hand out of a moving car is excellent for explaining stalls, it's not so great for explaining Bernoulli's principle.

Oh, my bad. The question isn't really about Bernoulli.

with a bit of origami skills and some glue you can make a small airfoil you can use for demonstration

Eeek! I would not advise sticking any limbs out of the window.

Well, technically it is still Bernoullis principle, it's just not an efficient airfoil

@falstro: Many airfoils are symmetric! And most of the rest is only asymetric slightly. The reason hand is a relatively poor airfoil is that it does not have a sharp trailing edge.

@JanHudec i know that, i didn't say it was a poor airfoil due to being symmetric, i said it doesn't generate lift due to Bernoulli's principle. I might be under the wrong impression, but I always thought Bernoulli's had to do with pressure differences due to the shape of the airfoil, not the angle of attack.

@falstro: The Bernoulli's equation obviously holds around the wing just as it does everywhere else, but there is actually no obvious way to fix enough variables to calculate the lift from it. There are many other laws of physics coming into play here, but the easiest way to explain lift is that the air continues downward by inertia as it leaves the trailing edge and by law of action and reaction has to exert upward force  on the wing, which is why the sharp trailing edge pointing downwards is important. For detailed discussion see How It Flies.

@JanHudec; Ok. I'm not sure exactly what you're trying to say here, but feel free to drop by the chat and clear it up for me. It seems that you're asserting that my statement is wrong, and thus that Bernoulli's principle is indeed involved with generating lift on a symmetric airfoil facing an on-edge airflow. (Remember, arm out the window..)

@falstro Bernoulli's principle still applies to other objects. A flat board can generate lift, just not as well as a 747 wing. Take for example the instance of a plane flying inverted, the wing is completely backwards from it's intended use, but it works nonetheless!

@BrianWheeler; Ah, I see. Your argument is that Bernoulli's principle is the sole source of lift, and mine is that it isn't. I see I have some studying to do. I never said there'd be zero lift (actually, the very first comment stated that there'd be lift with a positive AoA). I am fully aware that anything can generate lift, and that many planes (aerobatic ones in particular) have symmetric airfoils; trying to convince me that symmetric airfoils can produce lift when I'm already saying that they can is what got me confused. :)

@falstro sorry I didn't mean to confuse anyone, however I believe that symmetric foils will produce lift using Bernoulli's principle. It is much more far-reaching than people give it credit for...

Though rather belated, FOR THE OP's Question:
  The wing's downward push on the air under it is a good start.  Pushing down on some bunch of air causes the wing to be pushed up. This is Newton #3 and what I call The Natural Phenomenon of "Paired Forces".
I use the example of pushing off from a boat to jump to the dock.  The boat then moves away.  You can even talk about loading the boat with more mass to reduce its movement to avoid falling in (as shown on many "funny" videos).  The more mass (of boat, or air) that is pushed, the less it has to be accelerated.   Watching tests and doing some calculations you can see this mass of air accelerated to about 30 ft per second for a large airliner. (F=MA)
You may also mention helicopter downwash, an ordinary propeller propwash, or a jet engine jet blast for examples of the "push on something and get pushed the other way" of Newton's #3. The explanation of pressures and pressure gradients around the wing to accomplish this gets longer than a 9 year old probably needs.
However, scientists discovered that the air moving over the top must curve around and follow that upper surface and this (details omitted) also causes more air to be pushed down and adds to the lift.  Detail explaining why the upper surface effect occurs gets more complex and it must be well understood before attempting to explain it and only if necessary. (The "longer path" does not cause it to speed up.  It has to do with accelerating air around a curve)  The student can accept that there is more that they won't understand and can be satisfied by a partial, but correct, explanation.
I'm not sure where that climbing part is going, but it is always the relative motion of the wing through the air (called "Relative Wind") that is important, not the climb angle. I believe you refer to the "Angle of Attack" (AOA).  This is the angle that the wing makes as it meets the relative wind. If the nose is pointed up, but it is flying more or less level, this is because more lift is needed, USUALLY at low speed, when airspeed, and therefore lift is lower.  At some point, as the angle of the wing points up too far, the smooth flow of air OVER THE TOP turns turbulent.  It swirls around instead of following the wing smoothly and this destroys that upper flow and a large part of the lift.  This is called "stall".  This is proof that it is not juswt the lower surface that produces lift, otherwise the loss of smooth flow over the top would be a 'don't care'.
Also, the fan over the wing is actually a GOOD idea.  This uses what is called the Coanda Effect and has been used on aircraft.  The first I was aware of, as a youth, was the F-104 with its tiny wings.  In order to reduce the landing speed (and actually make it landable -- no engine, no land---bail out) they directed some engine blast-air through carefully designed slots over the wing. It was called "Boundary Layer Control".
PLEASE, all readers, see these authoritative sources.  They discuss lift and you should be able to adapt parts of them and simplify as needed.
Peter Eastwell Bernoulli?
http://www.scienceeducationreview.com/open_access/eastwell-bernoulli.pdf
  As a teacher, he didn't like what he read and researched this to find the truth.
Weltner in PDF - "Misinterpretations of Bernoulli's Law"
http://user.uni-frankfurt.de/~weltner/Misinterpretations%20of%20Bernoullis%20Law%202011%20internet.pdf
I'd post more links, but I'm limited.  Google can find them...
Prof Babibski @ Cambridge has a youtube video, but you must get his slides from links under the video. XWdNEGr53Gw
His missing slides are at docs.google.com
/file/d/0B0JABuFvb_G_MkpBZHJmRGo3UkU/edit?usp=sharing
He also has a comparable paper on ww3.eng called "Senior-glider/howwingswork.pdf"
Anderson & Eberhardt AAPT paper: The "Newtonian Description of Lift of a Wing"-Revised 2009:  on  comcast.net/~clipper-108/Lift_AAPT.pdf
gifday.com has a great animated GIF of an A380 test allowing you to actually see the downwash: 2012/02/a380.gif
--
Regards, Challenger Learning Center Science/Technical Advisor
Steve N.

You can... and some do... but you will not go fast that way
Well actually, she is right: you can use an engine to blow at the wings and create lift that way. And that principle is used on some aircraft, like on the Lun-class ekranoplan.
...but only when taking off.
The problem is that a huge portion of the engine trust hits the wings, which then brakes the plane, because of the drag. The thrust you gain from the engine is therefore lost back on the wings. The plane is pushing against itself, so it does not get up to the speed that we want to.
So you can tell her: she is right, you can do it that way.
But you do not want to do it that way because once the plane goes fast enough that it does not need the "help" from the engines to stay aloft... then we cannot turn that off. And then her scheme just becomes a big brake instead, so the plane cannot go very fast.


text:
Why is engine No.2 started first, instead of No.1?
I realized that some Boeing manuals recommend that the engine number 2 must be started first, is there any special reason for this?

which boeing planes?

I think it has to do with what side the passengers load from. I think typically the engine on the side without the doors is started first, but don't have a source.

There is an interesting discussion here: http://www.airliners.net/aviation-forums/tech_ops/read.main/89869/ where they cite pilot location, APU and electrical configurations, etc.

The isolation valve in Open or Auto ensures that the Auxiliary Power Unit (APU) bleed air reaches the engine number 2 starter. Engine number 2 start Exhaust Gas Temperatures (EGT) are generally higher.
http://www.lt-vacc.org/images/mesti/Boeing%20737-300-400-500%20Operations%20Manual.pdf
Engine number 2 is started first because it's on the opposite side of the air bridge. When starting with external air or power it is normal to start Engine number 1 because it is further away from the carts.
http://theflyingengineer.com/projects/airbus-tech/hydraulic-systems/
According to Flight Crew Operations Manual (FCOM) 3 Standard Operating Procedures (SOP):

Engine 2 is normally started first. It powers the yellow hydraulic system, which pressurizes the parking brake

Looking at the electrical schematic that Boeing provides, we see that Transformer-Rectifier (TR) 3 is powered from the R/H AC Bus. TR 3 is an important TR from which significant avionics are powered. Ergo, you would want to have this TR powered up and running first in case you lose AC power to the aircraft during start.

Your answer would be of more value to non-experts when you expand it to cover some essential background information and explain the abbreviations.

@  Peter Kämpf - IIRC there is no such rule that states that Engine No 2 has to be started first . Some pilots do start Engine No.1 first . So , I am afraid that's the only background that I am aware of . Re abbreviations , I have amended it .

I will delete this comment if attended to: What is a TR? What is AC? What is an SOP? What is FCOM? etc. I am not an airline pilot. Only an enthusiast.

TR = Transformer-Rectifier;
AC = Alternating Current;
SOP = Standard Operating Procedures;
FCOM = Flight Crew Operations Manual;
The definitions of each one, you may be find at google, some are a bit complicated to explain here.

@YgorMontenegro with respect to AC meaning Alternating Current - it seems redundant to refer to it as AC power when you can just say power - whether DC or AC, power is power in my view - they are pretty similar.

In airplanes, some systems are projected to be powered with different kinds of currents, take a look in this manual and you will clearly see the difference: http://www.smartcockpit.com/aircraft-ressources/B767_Electrical.html

Because most people are right handed.
Let me expand a bit on that:
Ancient Boats
Our story begins in ancient times, long before aviation took off.
Early boats were steered with a steering oar attached to the side of the boat. You can see one in the following tapestry:

(image source: Wikimedia)
Since most people are right handed, the steering oar was attached to the right side of the boat. The Old English word
steorbord comes from this steering oar, from which the term starboard for the right side of a ship originates. When
docking at a wharf, the steering oar would be in the way. That is why boats always docked from the left side, hence called
the port side.
Modern ships use rudders on the centreline of the ship, so they can dock from both sides. The terms port and
starboard are however still used today.
Flying Boats
What does all of this have to do with aviation? Enter the flying boat:

(image source: Wikimedia)
In the beginning of aviation, most passenger aircraft were flying boats because very few airports existed at the time,
particularly ones with long enough runways for larger and larger aircraft. Flying boats could just land on a river, a
large enough lake or the sea. This is also the reason for why we are using many nautical terms in aviation (like e.g.
nautical miles for distances) and why pilot uniforms look like Navy uniforms rather than Air Force uniforms (see also
What's the reason for pilots wearing uniforms?).
How did these flying boats dock when they arrived at a port? Of course with the port side towards the port. That
is why passengers had to board the aircraft from the left side. This tradition lives on until today (see also
Why do passenger embark on the left side of an aircraft?).
Before Auxiliary Power Units (APUs) existed, you had start a main engine to get electrical power (without draining
the battery) and air conditioning. Since the passengers were boarding on the left side, you had to start a right
engine first. Once all passengers were on board and the doors were closed, the left engines were started as well.
Modern Aircraft
Today, most (maybe all) passenger airliners have an APU to power the aircraft (and start the engines). They can also
be powered by other means at the gate (see
What powers an aircraft prior to the APU being switched on?).
This means one could start either engine first today. Yet, it is still common to start the right engine first, e.g. the Boeing 737 FCOM, Normal Procedures, Engine Start Procedure says:

Normal starting sequence is 2, 1.

So why do we still care? In part, due to tradition. But there are some reasons to start the right engine first:

APU: The APU uses fuel and this cannot come from the center fuel tank because that could be empty (see
Why are fuel tanks in the wings filled first, and why are they used last?).
So the designers have to choose either the left or right wing tank for the APU fuel supply. Since it was tradition
to start the right engine first, they choose the left wing tank for this. So the APU starts to use some fuel from
the left and then when the right engine is started first, it will help to partially balance the fuel.

Electrical Power: DSarkar's answer claims that starting the right engine provides power to Transformer-Rectifier 3, which will power the AC systems. This is however not really important since the APU will power all AC systems during engine start anyway. The 737 does not even need any AC power when starting:

In the GRD position, the engine start switch uses DC power from the battery bus to close the engine bleed air valve and open the start valve to allow pressure to rotate the starter.

Hydraulic Pressure: DSarkar's answer further says that starting the right engine is important for providing hydraulic pressure to the parking brake on the Airbus A320. However, all Boeing aircraft can fully pressurize all hydraulic systems electrically (and some pneumatically) before the engines are started.

In the end, it really does not matter any more today. Any Boeing could safely start either engine first. As a matter of fact, sometimes the pushback crew actually asks to start the inboard engine first when turning the aircraft during pushback because they will have to overcome less torque that way.
Conclusion
And there you have it: we start the right engine first because passengers board on the left because boats
docked on the left because the steering oar was on the right because most people are right handed.
Bonus: Tail Mounted Engines
The tradition to start right engines first only exists for wing mounted engines. Aircraft with tail mounted engines
typically start the engines from left to right. That is why the APU typically uses fuel from the right wing tank
for these aircraft (e.g. MD-80, Boeing 717).

Reminds me of Why do the British drive on the left? (spoiler: it’s all to do with keeping your sword hand free!, so basically for the same reason).

Sometimes the most complex things have the simplest answers... Is there any documentation of any sort to back this up? I agree 100% (and +1) that it's very logical and makes a ton of sense. Is there something stating that the flying boats would start an engine before all passengers had boarded, thus necessitating not starting the one on the side where they were walking? That would take excellence to greatness! (or is it the other way around?)

@FreeMan This answer to a remotely related question says the order is 3-4-2-1, i.e. starboard first, inner to outer. They say it has technical reasons and is safer and more convenient for the pax because they board from the left.

@FreeMan I remember that I heard about this in a documentary about flying boats. I just tried to find it again, but couldn't. If I remember correctly, they also didn't start engines during boarding every time, but sometimes it was necessary to get electrical power or air conditioning running.

I think it may be a bit of a stretch to say that most early passenger airplanes were flying boats.

@quietflyer Well the bigger ones of that time were...

Such an interesting answer!  Can you quote your sources?


text:
What is the maximum taxi speed and who defines it?
Which is the maximum speed that an airliner is limited to while taxiing?
I know that during 90 degree turns it is 10 knots, but what about long, straight taxiways? 20 knots? 25? 30?
More broadly, where I can find this information?
Would it be in company SOPs? Manufacturer's FCOM?

Urban legend: According to Southwest's SOP, "anything below Vr"

@PhilippeLeybaert: would that not be V1? ;)

@Qantas94Heavy not if the next part of that sentence is "...in a Cessna 150".  :-D

A "Safety Alert for Operators" (SAFO 09004) from 2/11/09 says "Slow the aircraft to a fast walking speed on the centerline of the landing runway prior to attempting to exit the runway. Taxi at a fast walking speed until parked at the ramp or until aligned with the centerline of the runway for takeoff."
Which, of course, isn't a regulation, in this example is specifically talking about winter conditions.
Source: http://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/safo/all_safos/media/2009/SAFO09004.pdf
Riverside County in Southern California has an ordinance (regulation) that states:
Section 12.08.100 Taxi speed.
No person shall taxi any aircraft on the airport unless there will be no danger of collision
with any person or object. All aircraft shall be taxied at a safe and reasonable speed
commensurate with safe operation in relation to existing conditions and with due regard for
other aircraft, persons and property. (Ord. 5661 § 1, 1988; prior code § 5.11 (part))
Source: http://www.riversideca.gov/municode/pdf/12/12-08.pdf
So whatever a "safe and reasonable speed" is.  I've heard it called a "brisk walking pace."
I had an airline pilot as a ground school instructor once, and she said that ground control was giving them a hassle one day, so everyone decided to taxi at a "brisk walking pace" around LAX.  Got ground whipped into shape real quick!
The FAA's website, under "Best Practices" says to "Maintain an appropriate taxi speed."
http://www.faa.gov/airports/runway_safety/pilots/best_practices/
Since it's not clearly defined by the FAA, I assume that most airlines limit taxi speeds via their SOPs, also due to the variety of equipment, it might be unfeasible for the FAA to mandate a specific speed.
Roughly:

Max 30 kts: straight line with no close obstacles (for example: back-tracking on the runway)
Max 20 kts: straight line with obstacles (for example: on the taxiway, or close to other aircraft/stands/ground vehicles)
Max 10 kts: turns and entry onto ramp area

I can't speak for airliners, but for light aircraft "a brisk walking pace" (ok, maybe more like "a light jog") is probably the upper limit (and what's prescribed in my POH). If your airspeed indicator moves you are almost certainly taxiing too fast. (mumble mumblestupid downhill taxiway deltamumble mumblewreck my breaksmumble mumble)

If the runway/taxi way is icy, then I can see the "fast walking speed", however a "brisk walk" is about a 20 minute mile, which would be 3 mph.  Keeping in mind that a runway at a large airport can easily be 10,000 feet (almost 2 miles), that would be a 40 minute taxi just to taxi the length of the runway.  Nobody does that out there in the real world.

@lnafziger nobody does it in GA aircraft either (I have a mile-long taxi at my home field on some days and you can bet your bonnet I'm not doing that distance at "walking speed" unless conditions are lousy). My point was more that "appropriate" taxi speeds are an aircraft-dependent thing: you wouldn't catch me taxiing my Piper at 30 knots even if I was at JFK ('cuz a stiff breeze and I'd be airborne!), but an A380 taking a 3-mile taxi with nobody in front of them might be at 30+ knots for a good part of the trip and be perfectly safe doing so :-)

Only 30kts on the back taxi?  I think I've hit 75 before trying to get off the active with an approaching aircraft.  I'd say its all operator/ aircraft specific.

Yeah, that 'brisk walk' thing doesn't actually happen, except maybe in cases of extreme icing on the taxiway. Unless we're talking about a 'brisk walk' for a cheetah. For me, a normal semi-brisk walk is right around 4 mph. Land on runway 10 at KATL and that gets you an hour to taxi to the terminal. Not going to happen (though it may feel like it sometimes.) I think whoever came up with the 'brisk walking pace' thing greatly overestimated the actual walking pace of a human (or they weren't referring to a human.)

And here, a Reference from Boeing's 737 FCTM (pdf). Ch2 for ground operations.

You can find it here, enjoy: http://www.737ng.co.uk/B737%20Flight%20Crew%20Training%20Manual%20-%20All.pdf

It should also be noted that performing a "high speed taxi" on a runway can be up to V1. This is often used for testing equipment on a new aircraft or newly repaired aircraft.

Both of the Boeing 747 operators I flew for in the 1990s observed a max taxi speed of 25 knots on a straight taxiway. We were told in ground school that Boeing recommended 20 knots.
Concerning a previous answer of 10 knots in a turn, while that may be a good general answer, it would be subject to conditions. I remember once almost leaving the taxiway in a turn at Jeddah even though I was a little less than 10 knots. It was over 110F, which made the asphalt a little slippery. After than experience, I slowed my 90 degree turns to around 6-7 knots if there was any question concerning the conditions.
Another consideration concerning taxi speed is how heavy you are and how far you have to taxi. The problem is tire heating. I seem to remember being told the Boeing 747 had a maximum taxi distance of 30,000 feet at maximum weight. I don't know whether that was really true, but I do know that at Honolulu a taxi to the reef runway from some parking areas is at or a little over that.
Since I've been retired for 15 years, please take anything I say with due consideration to my ageing-failing brain.

Any kind of information is welcome! Many thanks for your answer Terry!

Tire sidewall heating is also the limiting factor for automotive and truck highway tires, either in speed or in load. Essentially it is the amount of flex (load and air pressure) times the number of flexes (rotations) minus the heat dissipated (time based); So fast and light or slow and heavy are ok, but not fast and heavy. Very high performance tires come with a de-rating chart that gives acceptable combinations of gross weight and max sustained speed. I forget how many minutes is considered "sustained".

There is no official FAA guidance on taxi speed.  There is an urban legend around that the speed limit is "no faster than a man can walk" and although every airplane exceeds that pace today, the origin of that "no faster than a man can walk" goes back to the early days of aviation.  The very early planes had no brakes and no steerable wheels!  They required wing walkers to stop the planes and to guide them around corners.  Yes it was possible for the pilot to raise the tailskid off the ground and rudder his way around a corner, but in close quarters this would add too much speed so that wing walkers were the solution.  Why the rule of "no faster than..."  Don't leave your steering gear (the wing walkers) behind!

This explains why when I measured, got values more like "no faster than a sprinter can run"


text:
Can you tell ATC you are a "student pilot" when you are not?
Many pilots who are not native English speakers have a hard time when they start flying in the U.S. because of the pace and sometimes elaborate instructions given to pilots by controllers.
The advice given by instructors is to tell the controller that you're a student pilot, even though you may be a fully licensed (private) pilot. The effect should be that the controller will be more lenient towards the pilot and speak slower.
I would first of all like to know if it is legal to tell a controller that you're a student pilot even if you're not (I know, all pilots should see themselves as students for the rest of their life, but that's another topic).
Also, isn't there a chance that you'd get refused entry in class B airspace because you're announcing that you're a student pilot?

My gut says "yes, it's legal; no, you won't be refused entry for that reason", but this is a weird one, and my gut also says "this is a weird solution to a language problem that should be solved in other ways, and you shouldn't do it".

One of the "Other Ways" I'd recommend: Put in some time as a VATSIM Air Traffic Controller. You'll be on the "wrong side of the mic", but you'll get to talk to a lot of pilots and practice both your English and standard ATC phraseology. I can also heartily recommend reading the AvWeb Say Again? column archives -just for the "inside look" at the ATC system.

I would also fly (non-SIC) right seat with someone else, and handle the radios on every flight. That way you can get lots of radio time, but without the added burden of PIC/sole manipulator.

@voretaq7 The URL you gave in your comment might be 404 and perhaps you should want to replace it with http://www.vatsim.net/air-traffic-control

Not as good as @voretaq7 's suggestion, but still complementary: You may listen to live ATC feeds.

When I was learning way back in the 80s in the UK, I read that one could use TYRO as a call sign prefix to indicate that you were inexperienced, even if not strictly a student. Never heard anyone do it.

"Low time pilot".....

@voretaq7 thank link is now dead

It's a bad idea to give false information to a controller as a matter of practice. Instead of calling yourself a student when you're not, it would be better to say you're unfamiliar with the area or that your English is not perfect.
Controllers are almost always accommodating to less-than-proficient pilots and will usually make allowances whenever time and traffic permits.
If your English is not good, be sure and use simple, direct phraseology. Read the AIM to become familiar with the common phrases used with ATC. Even something as simple as hearing "radar identified" in Canada instead of the usual U.S. phrase "radar contact" has caused many a native English speaker to reply "say again?"

Yes. This. This is the best answer possible, having taught students for whom English was not their primary language. Controllers want to help. Very, very rarely have I interacted with ATC in a way that did not make that clear. Tell them what you need from them, even if it's simpler or slower phraseology, and they will almost all do what they can.

What is the best thing to say to a controller when you just can't keep up with him/her? Or when they start giving instructions using local landmarks you've never heard of? BTW: I have the most trouble keeping up with female controllers :-)

Just repeat "Say again" or "Can you please spell that identifier?" until you have it. Don't be embarrassed about it if you don't understand -- your life may depend on it. They are as interested in getting you the message as you are in receiving it, so once they understand the they're not getting through, they'll slow down.

@PhilippeLeybaert - I see you're from Belgium. By coincidence, one of the the most difficult times I've had understanding ATC instructions was on an instrument approach to Charleroi Airport (a female controller). I was not as prepared as I should have been and did not recognize a waypoint identifier.

Yeah, for some reason in Belgium VOR waypoints are not spelled out. They mostly use their "real" name like Gosly for "GSY" in Charleroi, Costa for "COA" near the coast or Nikki for "NIK" near Antwerp. I could go on. Very confusing because these waypoints are only depicted by their 3-letter identifier on charts.

@PhilippeLeybaert ATC in the US does basically the same thing, though at least our charts have both the name and the identifier on them.

"Cleared direct Crazy Woman"

@PhilippeLeybaert when you can't keep up with the controller say, "SPEAK SLOWER". See my answer.

Is it legal? Sure.  As others have pointed out it's also pretty darn honest!
I'm a private pilot. I'm also a student pilot, working on my instrument rating.
When I'm done with that I'll probably work on a complex endorsement and a commercial rating, maybe even a CFI if I get ambitious, and a tailwheel endorsement is on my list.
If I can find a rich benefactor to pay for it a multi-engine rating or even a seaplane rating would be fun to pursue. And I'll sell my soul for a DC-3 type rating.
Even if I collect all the ratings I want I would still go up every month or so with an instructor just to kick around and drill stuff that I wouldn't normally do, and I work through phases of the FAA WINGS program, which is certainly designed as a training/learning experience.
Might a controller deny you clearance into specific airspace if you tell them you're a student? Sure.
But they might deny you the clearance anyway - maybe they're busy, or maybe they're just in a bad mood.

Does it matter?
I don't think so. I identified myself as a student pilot maybe five times in my flight training, all while doing required "supervised solo" work at the local airport.
In my case I've always been comfortable with radio communication, so it wasn't an issue from that standpoint, and I've never felt that I needed any "extra patience" from ATC - they tend to be a very patient lot, even when I screw up (a few legendary local grouches aside).
Remember:

I can't agree that a "student pilot" is one who has a private certificate but is working on another rating or commercial, based on http://www.law.cornell.edu/cfr/text/14/part-61/subpart-C . It is a misrepresentation of the responsibilities and privileges of your certificate. Imagine what would happen if g-forbid you had an emergency, and then said "three souls," when its illegal for you to carry pax?There is other phraseology which will convey the difficulty you are having without misrepresentation, and that would be "unfamiliar" and to do a radio check.

@rbp Are you sure its legal to carry passengers on an IFR (training) flight plan while not an IFR rated pilot? I can't find a FAR on it right now but I'm guessing the answer is no...

who said IFR???

@RonBeyer I'd say you can: The answer would be closely related to whether you can legally file IFR without an instrument rating: If you can't then either (a) you're operating a VFR flight and not a "student pilot" in the part 61 sense - you're a private pilot acting as PIC; or (b) In IMC your instructor is onboard, their name is on the flight plan, and they're legally PIC "final authority and responsibility for the operation and safety of the flight" even though the instrument student is likely "sole manipulator of the flight controls".

A licensed private pilot seeking an additional rating is not a student pilot and may not follow the regulations relating to student pilots.

@DavidSchwartz The only regulations relating to a student pilot’s flight are some additional restrictions. Can I follow the regulation that says student pilots must have 3 miles visibility? Sure. In fact my personal minimum is way higher than that. Not fly to any airport unless an instructor has flown there with you and signed you off? Sure. Get specific training before flying in Class B airspace. That’s actually a good idea, whether it is with an instructor or more experienced pilot. That’s what I did. I am not aware of any additional privileges that student pilots have that a PPL does not.

@JScarry Try acting as PIC without having had a BFR in the past two years and claiming that it's okay because an instructor signed you off for solo flight. You cannot both hold a PPL and follow the regulations for student pilots. Really. The regulations say so and the FAA says so. If you choose to do so, you had better be able to find other regulations that allow everything you do.

@DavidSchwartz Private pilots definitely have other requirements that student pilots do not have. e.g. Passenger carrying currency. I am not aware of any regulations that give privileges to student pilots that are not also available to private pilots.

@JScarry It seems like you're not reading what I'm writing. I specifically made you aware of such regulations and cited the FAA explaining it. For example, a student pilot can fly solo without a current BFR. And this isn't arbitrary or a quirk, there is a well thought out rationale for why, in some cases, a private pilot should be subject to a restriction that a student pilot is not (typically because the private pilot is not subject to other restrictions that the student pilot is and the result would be illogical if given the benefits without the restrictions).

There is always more that can be added to a topic, I'm answering late here, but there is one thing that no one has pointed out to my unpleasant surprise.
The Pilot/Controller Glossary!
The asker's question says: (My emphasis)

The effect [of declaring that you are a student pilot] should be that the controller will be more lenient towards
the pilot and speak slower.

Check the pilot/controller glossary and find the bolded terminology "SPEAK SLOWER."
The bottom of this page in this .pdf file of the pilot/controller glossary is the term.

SPEAK SLOWER.  Used in verbal communications as a request to reduce speech rate.

The AIM 4-2-4 says:

c. Student Pilots Radio Identification -

The FAA desires to help student pilots in acquiring sufficient practical experience in the environment in which they will be required
to operate. To receive additional assistance while operating in areas
of concentrated air traffic, student pilots need only identify
themselves as a student pilot during their initial call to an FAA
radio facility.

EXAMPLE:
DAYTON TOWER, THIS IS FLEETWING ONE TWO THREE FOUR, STUDENT PILOT.

This special identification will alert FAA ATC personnel and enable them to provide student pilots with such extra assistance and
consideration as they may need. This procedure is not mandatory.

My answer here is, yes; it is legal to say you are a student pilot even if you aren't, but why do that when you can say, "English isn't my first language. Could you please SPEAK SLOWER for me?"
Also: no, ATC would not deny you entry into class B for saying you're a student.  They may follow-up by asking if you have the required endorsements from your instructor to enter class B.  Then, if you tell them you aren't actually a student, you may just get on their nerves.
Controllers are human, just tell them what your needs are and they will help you.  Read the pilot/controller glossary.
Another bolded term in the pilot/controller glossary for those who may need extra assistance in communication is "WORDS TWICE."
WORDS TWICE:

a. As a request: “Communication is difficult. Please say every phrase
twice.”
b. As information: “Since communications are difficult, every
phrase in this message will be spoken twice.”

So to add to the above, "English isn't my first language. I request that you SPEAK SLOWER and say WORDS TWICE for me."

I would like to emphasize; technically legal since I am not aware of any explicit rule that says otherwise, but strongly not recommended.

An experienced ATC controller giving my flying club a presentation suggested that as you said, we're all students, since we are always learning.
His recommendation is to say 'student pilot' whenever we feel like we need extra patience and clarity regardless of how long we've had our PPL.
That said, I wouldn't over use it unless I really felt like I needed some extra patience, especially in a class B.

Nice answer based on real-world input--

I am guilty of suggesting to students that if they get into trouble while flying to say "student pilot" because that always gets them kid glove treatment.  Controllers can be great.  The problem is that over the years I have seen pilots investigated for a variety of things, and it doesn't look good when you misrepresent yourself to ATC.  Review boards don't like pilots misstating facts.
So my opinion has changed.
Perhaps "unfamiliar with airport" or some similar phrase might be better.
Addendum #1
I have a new opinion on this matter.  After discussing it informally with an FAA attorney, he suggested that a private pilot using the terms "student pilot" could be good cause for a 709 ride, since it is an admission that they lacked the skills to exercise the privileges of the certificate they held.  FWIW, that FAA counsel is also a CFI-AI, and understand well practical issues of getting help at a strange airport.
So my current take is to never use the term "student pilot" except prior to holding a certificate higher than a student pilot certificate. In other words, don't misrepresent yourself.

How does saying that you are "unfamiliar with [the] airport" prompt the controller to speak more slowly, as OP wanted to begin with?

"Speak slowly please" works in my circles.  Saying "Student Pilot" misrepresents your status.  Just landed at DCA, and I tell the controller, "Student Pilot" doesn't work well.

The answer to your question can be found in 91.123:

§ 91.123 Compliance with ATC clearances and instructions.
(a) When an ATC clearance has been obtained, no pilot in command may
  deviate from that clearance unless an amended clearance is obtained,
  an emergency exists, or the deviation is in response to a traffic
  alert and collision avoidance system resolution advisory. However,
  except in Class A airspace, a pilot may cancel an IFR flight plan if
  the operation is being conducted in VFR weather conditions. When a
  pilot is uncertain of an ATC clearance, that pilot shall immediately
  request clarification from ATC.

Telling the controller you are a student pilot when you are not is not an acceptable way to "immediately request clarification" from ATC when you are uncertain of your clearance.
Imagine you're a JAL pilot flying a 777 into JFK and you have a problem understanding the controller from Queens. Would it be acceptable to say "Student Pilot?" Of course not, because its a lie, and its still a lie if you're banging around in a 152 under part 91 holding a private pilots license (or better).

There is a legitimate term for a novice pilot that is not a student, and the term is TYRO. You would use it as a prefix in the same way you would STUDENT.
I've encountered it in the UK, but it's not common even there. I don't know if ATC would recognize it. It has military origins.
For more info see: What is the origin of the term "Tyro" for inexperienced pilots?

This is probably technically a crime under 18 U.S. Code § 1001:

whoever, in any matter within the jurisdiction of the executive, legislative, or judicial branch of the Government of the United States, knowingly and willfully [...] makes any materially false, fictitious, or fraudulent statement or representation [...] shall be fined under this title, imprisoned not more than 5 years [...] or both

Air Traffic Control is a matter within the jurisdiction of the executive branch. People have been convicted under 18 U.S. Code § 1001 for lying to the FAA before. Are you likely to be sent to prison for claiming to be a student pilot when you are not? Probably not. But it's one more reason besides the common sense "lying is bad" and "why would you lie when you can just ask ATC to speak slower?" to avoid lying to ATC.
You certainly may be declined access to certain class B airspace if you identify yourself as a student pilot. While students can receive an endorsement to operate in class B airspace, certain airports are wholesale forbidden to solo student pilots regardless of endorsement. Currently these airports are KATL, KBOS, KADW, KORD, KDFW, KLAX, KMIA, KEWR, KJFK, KLGA, KSFO, and KDCA. Of course, if you are uncomfortable enough with ATC communications to consider identifying yourself as a student pilot, you probably should stay out of class B airspace to begin with.
Additionally, you will not be offered a LAHSO clearance. If you are carrying passengers and you have an emergency, it may create communications issues. Since you've lied and told them you were solo they may or may not ask how many people are aboard. You will need to retract your statement about being solo, and I don't think the FAA would be impressed if/when they investigate.

"Student pilot" and "student solo" are not synonyms, though. At least I don't think so...?

@randomhead Maybe? I don't think people normally identify themselves as a student pilot when they have a CFI on board. The LAHSO order says "When pilots identify themselves as a solo student pilot, that pilot must not be issued a LAHSO
clearance." But the phraseology given in the AIM is just "student pilot," not "solo student pilot" or "student pilot with CFI." I assumed the solo was implied.


text:
What type is this aircraft, appearing in the movie "Kingsman"?
In a scene of the movie Kingsman: The Secret Service the young aspiring agents have to skydive from an airplane. I didn't recognise it: which airplane is it (assuming it actually exists)?
I could only find a picture from inside and one from below.

Fortunately the one from below includes a (barely legible) registration.

To follow up with that, here's more photos of that actual aircraft: http://www.airliners.net/search/photo.search?regsearch=G-BEOL

That distinctly squarish shape looks like a Short SC-7 Skyvan 3-100, especially since the registration on the photo from below is visible: G-BEOL.
 -

(Image Source: WikiPedia - Author: Adrian Pingstone)

Youtube: Short Skyvan flight over Spa for parachutist dropping

Indeed, this is a Skyvan. It was built in Belfast at the Short Brothers facility, near to the then RAF Sydenham - now George Best Airport. My father worked for Shorts as a senior design engineer. In the late 1960s I was able to visit the assembly floor. I was aged nine or ten. I saw part-completed early variant Skyvans. from time to time, I saw the finished planes over Belfast Lough as they were subjected to test flights.

It is the Shorts SC-7. You can see the short nose compared to the C-23 Sherpa. I too thought C-23 because I worked on them in the US Air Force.


text:
Does asking for deviation due to weather give ATC any leeway for reducing separation between aircraft?
When needing to deviate due to weather, does asking for deviation "due to weather" give ATC any leeway in terms of aircraft separation?

No, it does not change the rules for aircraft separation. That's the whole point of them asking ATC for a deviation in the first place. See this question for the details about separation: http://aviation.stackexchange.com/questions/2806/how-much-is-the-minimum-safe-distance-between-two-planes-in-flight/2809#2809

No, it does not. ATC can specify a restriction to the deviation, especially in a non-radar environment where they can't keep an eye on you.
The only thing that frees ATC of separation responsibilities between two IFR (or IFR/VFR depending on airspace) aircraft is if one them reports the other in sight, and is issued an "maintain visual separation". After that the responsibility of not hitting the other aircraft, or its wake, rests solely with the pilot in command.

Aircraft deviate for weather without radar all of the time (think oceanic flights).  What are we supposed to do, just fly through that level five thunderstorm if radar goes down?  ;-)  That being said, I agree with the rest of your answer.  Separation is still required at all times (and that includes visual separation).

@Lnafziger that is true. I was thinking about that as well. How is that handled? Using corridors of certain size? How about more congested airspace when the radar is down?

Timing, pilot position reports, etc.  We need controllers here to answer these types of questions!  :-)

@Lnafziger I'm guessing timing and position reports only really work if you have a route, so... re-routing then?

Well, I've been asked to report my bearing and distance to a VOR when I'm in a non-radar environment (which is what I meant by position reports) but typically they just say something like "10 degree right deviation approved, cleared direct  when able.", sometimes with a restriction to not deviate more than a certain number of miles of course.  They would then need to keep all of that airspace clear.


text:
Should I fly if conditions are VFR but the flight briefer says "VFR Flight Not Recommended"?
I contacted a flight briefer this morning in the San Francisco Bay Area in California. At the time, ceilings at my airport (Palo Alto) had just hit 1000 ft, so the field was (barely) VFR. It is a very common pattern in this area for the ceiling to burn off, become broken, scattered, and then disappear.
The briefer told me there was an Airmet Sierra for IFR conditions and mountain obscuration in force, and that VFR flight was not recommended. When I talked with an instructor at the club, we talked about the specific location of the clouds and decided the whole area would soon be VFR.
Question: should I fly if conditions are clearly VFR but the flight briefer is telling me “VFR flight not recommended”? How should I think through the risk assessment here?

the SF Bay area gets a marine layer (stratus) many days out of the year that can stretch as far as the East Bay Hills and beyond. Most days, as the sun heats up, the marine layer burns off or retreats towards the coast. Briefers for that area will always say VFR Not Recommended, but if pilots abided by that, nobody would be able to fly for much of the year.

...The thing to be careful of in the Bay Area is when the air starts to cool again in the late afternoon -- sometimes as early as 4PM -- the marine layer comes back in, and you can be out of luck trying to gett back in VFR. I've had to come back in IFR (sometimes hard to get a popup) and cancel underneath the layer and fly to my destination airport, and I've also scud run, but only because I know the safe way back in and the safe altitudes (eg the Sunol Pass).
http://en.wikipedia.org/wiki/San_Francisco_fog

"VFR Flight Not Recommended" is a judgement call made by the flight briefer. It means, as you noted from the AIM, that the briefer doesn't think flying around VFR is a smart idea based on what they're seeing.
The usual reasons for a briefer using this phrase are:

The weather is currently below VFR minimums somewhere along your intended route.
The weather is currently "Marginal VFR" somewhere along your intended route.
The weather is forecast to be marginal VFR or below somewhere along your intended route.
Some other adverse condition (like mountain obscuration) exists along your route or in the area you intend to fly.

The briefer's recommendation is just that: a recommendation -- it is not binding. How you evaluate the flight to make a Go/No-Go decision when the briefer provides this recommendation is your choice as pilot in command. You will need to consider why the briefer is telling you VFR flight is not recommended, and apply your own knowledge and judgment.
A couple of examples:

If the briefer is telling you this because Airmet Sierra is active for mountain obscuration and you intend to fly through some mountain passes prudence suggests that proceeding VFR into mountains that may be obscured by clouds would be a Bad Idea.
If the briefer is telling you this because the weather is currently marginal VFR, but it's forecast to improve and you only plan to shoot a few touch-and-go landings at your home airport you might proceed with the flight anyway.

For evaluating the huge swath of conditions in between those two extremes consider your familiarity with the area (typical weather patterns, topography, obstacles, etc.) and the conditions of flight (especially day-vs-night). Thomas P. Turner's site has a nice set of guidelines which I happen to broadly agree with, but each flight needs to be carefully evaluated to determine if you believe you can complete it safely under the conditions reported to you by the briefer.
The safest approach is obviously to cancel the flight (or complete it under instrument flight rules) if you have any doubt.

Additionally, the briefer may or may not be geographically located in the same place as the pilot calling the briefer.  So the briefer is looking at satellite pictures, and other reported information that may not be up-to-the-minute current, and he is making the recommendation based on that. If you're at the airport, you have the benefit of looking out the window and in the direction you're going to fly.

Quoting AIM 7-1-4:

2. VFR Flight Not Recommended. When VFR flight is proposed and sky conditions or visibilities are present or forecast, surface or aloft, that, in the briefer's judgment, would make flight under VFR doubtful, the briefer will describe the conditions, describe the affected locations, and use the phrase "VFR flight not recommended." This recommendation is advisory in nature. The final decision as to whether the flight can be conducted safely rests solely with the pilot. Upon receiving a "VFR flight not recommended" statement, the non-IFR rated pilot will need to make a "go or no go" decision. This decision should be based on weighing the current and forecast weather conditions against the pilot's experience and ratings. The aircraft's equipment, capabilities and limitations should also be considered.

A 1000 feet ceiling means you can only climb to 500 feet AGL to stay VFR.  Maintain that altitude, which on the coast is close to 500' MSL, proceeding inland the ground is climbing rapidly towards you. Rapid deceleration upon contact with the ground might be fatal.  VFR not recommended.


text:
Is it required to contact approach when flying VFR into a Class D airfield?
Is it required when under VFR to contact an airfield's approach when approaching to land at a class D?
KDLH is my example airfield. They possess independent radar approach and departure (same frequency) with a separate tower frequency. What would be the process of approaching this airfield under VFR?

No it is not required to contact approach. Unless you have to go through a larger D, C or B airspace to get there. I don't know of any larger Ds in the US, but there are plenty of D-airfields underneath C and B airspace.
In Germany (and I think around Europe, not sure though), there's a distinction between the tower-D and the approach-D (the former being called D-CTR, and the other just D), to enter the approach-D you'd have to talk to approach, but if you can sneak under/around it, go ahead and call the tower directly, let them know where you are, and they'll tell you how to enter their airspace. In the US, they'll probably just tell you to enter the pattern (like "N12345, enter and report the right base for runway 5").
In Europe you need an explicit clearance and they'll usually tell you how to enter (like "DEFGH, enter control zone via Echo, report Echo", Echo being a reporting point on the chart, Echo usually being to the east, with November, Whiskey and Sierra being north, west and south respectively (usually, that is, there may be more, and they may be placed differently, but they're always on the sectional or the terminal area chart)

To be honest, I've never heard of tower-D and approach-D. In the countries I've flown (in Europe) the airspaces were named CTR (around airports), TMA (shelves above a CTR) and CTA (above that). Anytime there's a TMA associated with a CTR, you have to talk to approach first, unless you stay below the TMA and want to enter the CTR directly.

@PhilippeLeybaert; Ok, well, in Germany they're called D-CTR and D (mind you, there's also C, but no C-CTR, so all surface airspace is either D, F or G), in Sweden they're called CTR and TMA as you say, however they still have an airspace classification (which in Sweden is C and not D, IIRC), so you can't tell from the classification (which the question is in regards to) if you need to talk to approach or not.

@Pondlife I suppose it's possible there's an airport somewhere with Class D airspace that REQUIRES you to call approach before you can call the tower, even for VFR flights, but I can't think of any off hand. That would certainly be an unusual situation though: I'd expect a NOTAM about the procedure at the very least.–  voretaq7Dec 25 at 7:05

Required is too strong a word.  Standard Procedure to land VFR at Westchester County Airport, White Plains NY (KHPN) is to contact NY Approach first for sequencing.  However, I have heard both sides of this: a pilot being chewed out for not contacting approach, and sent back away from the Delta space to "get in line", and also the tower accepting arrivals that just call the tower as the first contact.  It is all based on how busy the Tower is.
The "requirement" to call Approach first is not written in any NOTAM I can find.  It is a "local knowledge" thing.

I suspect that it's the same kind of "local knowledge" as Republic (KFRG)'s "voluntary" noise abatement procedures - they're technically voluntary and we get some transient pilots who don't look them up, but when they don't follow them the folks in the tower certainly sit up and take notice of the airplane "behaving strangely" and may ask pointed questions like "N12345, Where are you going?"

Out of courtesy, you MAY contact approach when VFR, but it is by no means necessary. Judging by the 86th issue of the Green Bay sectional, it is not even obvious that radar services are available.  The procedure would be to listen to ATIS on 124.1 and contact tower before entering the class D.
I believe this is true even if you are flying into a class D airfield with charted TRSA, such as Fargo (FAR) or Tri-Cities (TRI)  You are welcome and encouraged to lookup the frequency on the edge of the chart, (it will likely be contained in the ATIS) contact approach during the approach to the field, but it is by no means compulsory.  You can make your desire for non-participation clear by stating "Negative Radar Service" in your initial contact, arriving or departing.

Contacting Approach is not required for VFRs unless the D tower’s ATIS says so.
For instance, while you won’t find it on the charts, A/FD or NOTAMs, the ATIS at KADS always says “contact Regional Approach on 124.3 and advise you have information [letter]” rather than the usual “advise on initial contact you have information [letter]”. If you try calling Tower directly, they will tell you to remain clear and contact Approach.
Regardless, any time Approach service is available, it is a good idea to use it. Not only will they usually clear you through any class B/C airspace above or around your destination airport, saving you time and money, but they can also merge you into the stream of IFR arrivals so it’s safer for everyone.

I was just about to answer this question exactly this way, Although, I have not run into another Class D airport that requires this in the Texas area. I imagine Addison can’t be the only one. Do you know of others?

@DeanF. ADS is the only one I know personally; their airspace and traffic density are ... problematic.

Unless there is a charted B or C airspace, you are not REQUIRED to contact the approach facility.  Different towers may have different opinions on what they prefer you to do, however.  Some have TRSAs (responsibility is mapped out, and you can participate if you want), and some just mysteriously have a approach/departure frequency listed and it's anyone's guess where that airspace extends to.
As a further amplification, they may just tell you 'contact approach on 123.45 and remain clear of the delta'.  So, their own operating procedures will dictate that you do contact approach, but you could run into a similar situation at any airfield where you just have to follow tower instructions.


text:
How can pilots fly VFR over-the-top in overcast conditions?
It's apparently legal for pilots to fly over the top of clouds and fly VFR. However, I don't understand how it's possible to do so, especially since there is no visual reference to rely on to ensure that you are heading in the right direction. So, how exactly does this work, are there any limitations on this and is it possible to be done safely?

Student pilots in the US are not allowed to fly over-the-top. §61.89   General limitations (a) A student pilot may not act as pilot in command of an aircraft: (7) When the flight cannot be made with visual reference to the surface;

Take a common situation hereabouts: fly from someplace in the clear SF Bay Area to Reno/Tahoe (or vice versa), travelling over the fog/low cloud obscurred Central Valley.  You have a compass, VORs, GPS, and most of all, you can see the moutains on the other side of the valley.  So you just point the nose at the appropriate mountain :-)

Perhaps the most important rule for a VFR pilot is "see and avoid" -- be able to see any immediate dangers (other aircraft, buildings, ground) and avoid them. When flying above the clouds, you can certainly see other aircraft and avoid them (as long as you're maintaining required cloud clearances), and the same for buildings. You cannot see the ground but on the presumption that your altitude is well above the ground and not significantly changing, this is not a factor.
As long as you're able to see the horizon, you're unlikely to become disoriented to the point of not actually knowing that the aircraft is or isn't flying straight and level. (Contrast that with flying in the clouds, where it's easy to become spatially disoriented and not be aware that you're starting to dive.) Limitations are basic: it's the PIC's responsibility to follow the regulations.
As to heading in the right direction and navigation in general, VFR flight does not mean one cannot rely on radio aids to navigation. VOR and GPS certainly make navigation very possible while on top, but to get to your question of "can it be done safely?" -- I suppose this comes down to a question of how reliable are your electronics and where are the weak spots; is the pilot under flight following or otherwise in contact with ATC, etc. If one were on top and were to lose the electrical system, there's certainly an emergency to be declared. If already in contact with ATC, it's an emergency that can likely be managed with the utmost of safety.

There are variations between states and types. E.g. in the UK, amongst other criteria, helicopters must remain in sight of the surface which severely limits the options to fly on top.

I wouldn't say that loss of the electrical system is an necessarily an emergency.  There are still airplanes that don't even have an electrical system that fly VFR (even on top).  They can use VFR GPS's or even time/speed/distance calculations to get them to a point where they do have ground reference.  I think that the safety aspect is more about weather at destination and whether or not you could fly IFR if needed.

@Simon - are you talking about VFR rules only for helicopter?

@BurhanKhalid Yes, sorry, I should have made that clear.

It should be noted that in some cases it's not allowed. In (most of?) Europe, flying in class F or G airspace at or below 3000ft AMSL or 1000ft AGL (whichever is higher), you are required to be "Clear of cloud and with the surface in sight."  Also cloud tops are not always aligned with the horizon (IFR pilots know this) so to keep a straight and level flight you should also double-check the artificial horizon.

The only real hazard of VFR over the top is a forced landing, engine failure being the most obvious. If the cloud layer is broken and the ceiling a few thousand feet agl then this is not a huge risk factor. If solid overcast you will need basic attitude instrument proficiency (now taught as part of most private certifications) to let down, otherwise low risk. If overcast and the ceiling is low or it is a fog layer, then high risk, instrument training and an IFR equipped airport in glide distance is the only recourse.(for fog, it needs full ILS or LPV glide path, ALS lights are good too.)

"in sight of the surface" does not mean directly above, a mountain in the distance is the surface and in sight. Though remaining within glide distance of the visual reference may be a stronger argument.
In the USA students are only limited while solo, VFR over the top is allowed with an instructor.

I've flown VFR-on-top plenty of times.  Navigation is simple: even the most basic airplane is required to have at the very least a compass, a clock and an airspeed indicator, which can be used to navigate much more accurately than you'd expect, given a reasonably good "winds aloft" weather report.  Most airplanes also have some kind of radio navigation equipment as well.  Furthermore, just because you are flying on top of a cloud layer, that doesn't necessarily mean you have no visual references for navigation.  For example, mountains may poke through the cloud ceilings, or there may be an area of fog covering a portion of your route, with clear skies at your departure point and your destination.  On one flight, I flew "VFR-on-top", but could see through the clouds to the streetlights below (it was dusk) even though my home airport was reporting IFR conditions (1/2 mile visibility, ceilings at ground level -- but it was a very thin layer of clouds).
Whether or not it can be done safely depends upon the circumstances.  Getting lost typically isn't the issue, but rather whether you'll be able to descend in VFR conditions to land at your destination.  Check the weather at your destination, paying attention to temperature and dewpoint spreads, and make sure you've got other options within the range of your aircraft.  Also keep in mind that while you're on top, if you have an engine failure, you'll be making an engine-out IFR descent into unknown terrain below, unless you are within gliding distance of a suitable landing spot in clear weather nearby!

Welcome to Aviation SE! Thanks for the answer.

Let us be very very precise. Are you talking about flying VFR-on-top, that is, you had an IFR clearance and were navigating solely by reference to instruments, and then the controller authorized you to fly "VFR-on-top"? Or were you flying VFR over the top, not on an IFR clearance, and were navigating by dead reckoning or occasional ground reference?